The present invention relates to rock reinforcing systems and, more particularly, to roof bolting methods and apparatus wherein a bolt is tensioned by threading into an anchor which is secured within a drill hole by resin grouting.
Among the various types of mine roof support systems employing rock bolts anchored in bore holes are the purely mechanical or point-anchorage systems, the resin grouted systems, and combinations of the two wherein the bolts is tensioned by threading into a separate member which is anchored in the drill hole by resin grouting. In the usual mechanical anchorage systems an expansion anchor is placed on the threaded end of the bolt prior to insertion into the bore hole. As the bolt is advanced, the anchor expands to grip the inner surface of the drill hole and the bolt may be tensioned to the desired degree by applying sufficient torque. Although resin grouting systems wherein the bolt is anchored directly by a hardened resin mixture offer certain advantages, the bolt cannot be tensioned once the resin has set.
More recently roof support systems have been proposed wherein the advantages of the mechanical and resin anchorage systems are combined. Such systems are typefied by U.S. Pat. No. 3,877,235 of Hill and U.S. Pat. No. 4,051,683 Koval, among others. In these systems an internally threaded member such as a nut or hollow collar is anchored within the drill hole by the resin mixture and the bolt is tensioned by threaded advancement into the anchored member. The internally threaded member is affixed to, or a portion of, an elongated bar which is surrounded by and anchored in the resin grouting. Stop means are structurally associated with the internally threaded member to limit the extent of threaded advance of the bolt. When the elongated bar and internally threaded member are anchored by the resin grouting and thus restrained against further rotation, the application of excess torque to the bolt serves to fracture the stop means within the internally threaded member and allow tensioning of the bolt.
Although performance of prior art roof bolting systems of the type outlined above is generally satisfactory, there remains the possibility that the stop means, being positioned at least partly within or upon the internally threaded member, may interfere with threaded advance of the bolt by damaging or becoming lodged in the threads. Also, placing the stop means in structural association with the internally threaded member requires additional fabricating operations which may be inconvenient and add to the cost of the assembly.
It is a principal object of the present invention to provide a novel roof bolting method, and apparatus for its employment, of improved reliability while being convenient and economical.
A further object is to provide novel methods and apparatus for reinforcing rock formations wherein a roof bolt is tensioned in a resin-anchored, internally threaded member which require no structural alteration of the internally threaded member.
Other objects will in part be obvious and will in part appear hereinafter.